I have two questions:
1)If a gerund takes place after a preposition(right?), then why the latter sentence doesn't sound like this:
I'm looking forward to seeing you?

2) Until/Till are prepositions, right? Then why I've never heard anyone says or writes 'Until arriving, I'll clean the house'? Everybody says until you arrive. But gerunds take place after prepositions, so I can't get the hold of it.

Thank you.
I'm looking forward to seeing you is correct.

until is most often a conjunction. That's how it's used in until you arrive.

When used as a preposition, until usually takes a noun object, not a gerund, as in until sunset, until the end of the year, etc. In the case of your example sentence, the noun form of arrive is more commonly used than the gerund: Until your arrival, I'll clean the house.

When a verb is used as the object of the preposition, it has to be in gerund form. You are right about that. But if the subject of that verb is missing, then it is taken to be the same as some noun in the clause it modifies. So Until arriving, I'll clean the house means Until I arrive, I'll clean the house (which makes no sense). Until arriving, I'll clean the house cannot mean Until you arrive, I'll clean the house.

Even though there may be a few cases where until is followed by a gerund, it's not usually very idiomatic.

Thanks a lot Jim, you're a great teacher.
BTW-Idiomatic is an ADJ?
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CalifJimI'm looking forward to seeing you is correct.
Then what about simply using the infinitive, not the gerund: I'm looking forward to see you.

Is this correct?
N5pn4cyaThen what about simply using the infinitive, not the gerund: I'm looking forward to see you.

Is this correct?
No, it's incorrect.
See also:

Just a comment adding to the experts’ post…. “looking forward to” is an idiomatic expression” What follows is either a noun, or noun phrase. i.e. I am looking forward to …What? What- implies a noun. My vacation in Hawaii, or my first dance lesson. Since gerund is a noun, it’s only correct form to use, not infinitive. Therefore, if you said “I am looking forward to meeting you, having my first driving lesson, or starting my new job. You are correct,

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Tanit, thank you so much for the informative links. I now know how to explain the same to others.
Goodman, thanks also for the additional info and interpretation.